1. Field of the Invention
The present invention relates to a color image forming apparatus using an electrophotography process or the like.
2. Description of the Related Art
An image forming apparatus configured to use an intermediate transfer member, such as a copier and a laser beam printer, is known. In such an image forming apparatus, in a primary transfer step, a toner image formed on the surface of a photosensitive drum serving as an image bearing member is transferred onto the intermediate transfer member by applying a voltage by a voltage source to a primary transfer member disposed opposite the photosensitive drum. In a full color printer in which a color image constituted by a plurality of colors is formed, a toner image constituted by a plurality of colors is formed on the intermediate transfer member surface by executing this primary transfer step with respect to each color and overlapping the toner images of the respective colors. In a secondary transfer step, a toner image of a plurality of colors which is formed on the intermediate transfer member surface is transferred onto the surface of a recording material such as paper by applying a voltage to the secondary transfer member. A color image is then formed by permanently fixing the transferred toner image to the recording material by a fixing means.
Japanese Patent Application Laid-open No. 2012-98709 discloses a configuration in which primary transfer is performed by using a belt-shaped member (referred to hereinbelow as “intermediate transfer belt”) as the intermediate transfer member and applying a voltage to a current supply member that is in contact with an outer circumferential surface of the intermediate transfer belt at a position which is set apart from the primary transfer region. In such a configuration, the primary transfer of a toner image from the photosensitive drum surface to the intermediate transfer belt is performed in image forming stations by using a secondary transfer member as the current supply member and allowing a current to flow from the current supply member to the intermediate transfer belt in the circumferential direction of the belt. Such a configuration makes it possible to remove a high-voltage power supply dedicated to the primary transfer from the apparatus configuration and reduce the cost and size of the image forming apparatus.
However, in the abovementioned configuration, in a monochromatic mode in which only a black station is used, a primary transfer current still flows in the photosensitive drums of color stations which are not used. As a result, the primary transfer current in the black station can be insufficient. Since the primary transfer is performed by allowing the secondary transfer current to flow in the belt circumferential direction on the intermediate transfer belt with respect to the photosensitive drum, a voltage cannot be switched independently for each image station. Thus, in the monochromatic mode in which a current flows also in the color stations which are not used, it is difficult to allow a suitable primary transfer current to flow in the photosensitive drum of the black station.